1. Field of the Invention
The present invention relates to a photographic printing apparatus which accurately measures the density of an image formed on a photographic film by a reduced number of light receivers and then prints the image on photographic paper.
2. Description of the Prior Art
Photographic printing is generally effected in such a manner that the light from a light source is passed through a negative film and an enlarging lens, and the image on the film is formed on photographic paper. The exposure time for a photographic printing operation is determined by detecting the quantity of light passing through a negative film by means of a light receiver.
One example of conventional practices is to dispose a light receiver on the side of a negative film which is closer to an enlarging lens and outside the optical path of exposure light in such a manner that the light receiver faces the negative film. In another example of conventional practices, a specular surface is provided on the upper surface of a shutter which is disposed between a negative film and an enlarging lens, and a light receiver is disposed outside the optical path of exposure light in such a manner as to face a specular image on the specular surface (see the specification of Japanese Patent Laid-Open No. 10943/1984).
However, the respective densities of various portions of an image on a negative film are not generally uniform, and the distribution of density may be distorted to a substantial degree. In such a case, the quantity of the light passing through the image on the film differs depending upon the direction in which the light quantity is measured. For this reason, to accurately measure the density of the whole of a film image, it is necessary to dispose an unfavorably large number of light receivers around the optical path of exposure light and to average the quantities of light which are respectively received by these light receivers.
Thus, a reduction in the number of light receivers disposed (e.g., one or two light receivers for each of the three primary colors) disadvantageously results in an inaccurate measurement of density, while an increase in the number of light receivers employed unfavorably leads to a complicated photometric system and an increase in the production cost.